Well the feats he accomplished, feeding on malachors dark side energies when it crushed kreia. Defeating malak when he was empowered on the star forge, note that malak was able to keep replenishing his energies and keep on fighting,

We also have the facts that the SF defences, droids and dark jedis were unable to stop him.

He also has vasts knowledge of the force surpassing an entire academys archives according to path of destruction.

For the force he is stronger than vader but in terms of pure lightsaber dueling abilities,id give it to vader in my opinion.

IN his holocron which bane studied also depicted him being able to create major force storms(weather) which can sort of cause massive damage over a certein area